Conventional passive infrared target scanning systems typically use two focusing lenses, a vertical scan mirror, a flat-faceted rotating regular polygonal mirror for horizontal scanning, and a single infrared detector, as shown in FIG. 1. The operation of a typical passive infrared target scanning system may be briefly described as follows. A target 10 will emit infrared radiation at an intensity which depends on, among other factors, its temperature. The radiation 12 emitted by a particular small area 11 on the target 10 is focused by a focusing lens 13 onto the vertical scan mirror 15. The radiation 12 is reflected by mirror 15 onto a facet 21 of a rotating, flat-faceted regular polygonal mirror 22, is then reflected by facet 21 of mirror 22, and is focused by focusing lens 26 onto infrared detector 33.
As is well known to those skilled in the art, vertical scanning of target 10 is achieved by moving the angle, .theta., of mirror 15 through a predetermined range. As is also well known to those skilled in the art, horizontal scanning of the target 10 is achieved by rotating regular polygonal mirror 22 at a predetermined constant speed, where mirror 22 has a plurality of facets 21, the plane of each facet being aligned with, or at the same angle with respect to, the axis of rotation 24 of mirror 22. Normally the plane of each facet 21 is parallel to the axis of rotation 24. Therefore, the sequence of operation in a typical system is: scan one horizontal line of target area 11, adjust the angle .theta. of mirror 15 to scan the next, usually lower, horizontal line of target area 11, scan the next horizontal line of target area 11, and so on, until the entire target has been scanned.
A typical passive infrared target scanning system will also require that the infrared detector 33 be maintained at a temperature of approximately -40.degree. C., and will therefore be enclosed in a cooling chamber 31 which has a window 30 made of a material which will pass infrared radiation 12 and still maintain the integrity of cooling chamber 31. Devices for moving the angle .theta. of mirror 15 through a predetermined range by a vertical scan motor 16 and a drive shaft means 17 are well known. Normally motor 16 is a stepping motor which incrementally rotates mirror 15 for each vertical step in the raster pattern. Likewise, means of rotating regular polygonal mirror 22 at a constant, predetermined speed by a motor 27 and a drive shaft means 23 are also well known. Motors 16 and 27 typically provide position information to a signal processor 35 over conductors 41 and 42, respectively. The output of detector 33 is provided to signal processor 35 over conductor 34. The outputs of signal processor 35 are provided to a display device 36 over conductor 37. Display device 36 then displays the perceived image 38 of target 10. The methods by which signal processor 35 processes the position information and detector outputs to produce a synchronized target image 38 are well known to those skilled in the art and will not be belabored here. The image 38 typically may be "black-and-white" (monochrome) or "false color". False color displays are ones for which predetermined colors represent various intensity levels of the output signal.
A typical passive infrared target scanning system suffers from several disadvantages: reduced performance if lenses 13 and 26 are not cooled because they will emit blackbody radiation which is detected by detector 33 and appears as noise to processor 35; a large cooling chamber or several smaller cooling chambers if lenses 13 and 26 are to be cooled; numerous windows 30 if more than one cooling chamber is used; and sophisticated means to maintain synchronization between the position of mirror 15 and the position of regular polygonal mirror 22.
While such systems work well and have proven their utility, their major shortcoming is that they are expensive. As noted above, this is due to the cooling systems required for elements in the optical path, and the expense of the precision parts used in the scanning system. Since a major part of the expense of such systems is the electro-mechanical scanning apparatus, there is a need in the art for a simpler, less expensive system for producing electro-mechanically implemented raster scans.